p-Dichlorobenzene is a raw material for polyphenylene sulfide (so called “PPS”) as an engineering plastic, which is industrially highly valuable, and its demand sharply increases in recent years.
Heretofore, a process for producing p-dichlorobenzene by chlorinating benzene or chlorobenzene with chlorine molecules in the presence of a Lewis acid catalyst in a liquid phase, has been known. For example, Patent Document 1 discloses that it is possible to produce p-dichlorobenzene at a selectivity of 75% by employing ferric chloride as a Lewis acid and disulfur dichloride as a promoter, and chlorinating chlorobenzene with chlorine gas while maintaining a reaction temperature of from 35 to 37° C. It discloses that, according to this production process, the conversion of chlorine is relatively high, but 25% of o-dichlorobenzene in industrially low demand is produced as a byproduct. Further, it has been known that a part of disulfur dichloride is reacted with e.g. chlorobenzene during the chlorination reaction and changes to diphenyl sulfide or thianthrene, and thereby decreases. Thus, the promoter degenerates during the reaction, and therefore it is difficult to recycle a catalyst in a system where disulfur dichloride is employed as a promoter. Accordingly, diphenyl sulfide and thianthrene are disposed of by washing the reaction fluid with water to remove ferric chloride, followed by distillation operation to separate them as a distillation residue. Further, this process separately requires a waste water treatment step.
Further, Patent Document 2 discloses a batch type process for producing p-dichlorobenzene, which comprises charging benzene or chlorobenzene, a Lewis acid and N-(chlorocarbonyl)phenothiazine to a reactor, and introducing chlorine gas to carry out chlorination. In its specification, there is a description that the selectivity of p-dichlorobenzene reaches approximately 82% by carrying out chlorination employing benzene as a raw material, antimony trichloride and ferric chloride as catalysts, and N-(chlorocarbonyl)phenothiazine as a promoter, while the reaction temperature is kept at 60° C. However, in such a method, chlorination activity is low, and the conversion of chlorine which can be calculated from the total amount of chlorine introduced and the composition of reaction fluid, is 90%, and a large amount of unreacted chlorine is entrained in exhaust gas (byproduct HCl). Accordingly, there are problems that in the case of industrialization, an equipment for purifying byproduct HCl is required, unit consumption of chlorine gas is deteriorated, and corrosion of an equipment tends to occur. Further, the reaction fluid is distilled, benzene is added to a distillation residue containing the catalyst and the promoter, and chlorination is carried out again by reusing the catalyst, but as the number of usage increases, deterioration of catalytic activity (increase of the amount of unreacted chlorine) and reduction of the selectivity of p-dichlorobenzene are somewhat observed. Further, the melting point of ferric chloride is high at a level of 282° C., and its solubility in benzene or chlorobenzene is also low, and therefore it is easily expected that the distillation residue is in a form of a slurry. When the catalyst is recycled, this residue tends to be attached to a removal pipe or a recycle pipe, and further when the attachment is washed, a large amount of benzene or chlorobenzene is required since the solubility of ferric chloride is low, and therefore it is industrially difficult to recycle this type of catalyst.
Further, Patent Document 3 discloses a batch type process for producing p-dichlorobenzene, which comprises charging benzene, aluminum chloride and a phenothiazine analogue to a reactor, and introducing chlorine gas while keeping a temperature at 50° C. to carry out chlorination over a period of from 2 to 3 hours, and it discloses that the selectivity of p-dichlorobenzene reaches approximately 86% when chlorination is conducted to the chlorination degree of 1.62. However, it failed to disclose the conversion of chlorine nor the total amount of chlorine introduced, and therefore the conversion of chlorine is unmeasurable and the reaction activity is thus unclear. Further, it is commonly known that aluminum chloride as a Lewis acid is instantly reacted with a trace amount of moisture dissolved in benzene as a raw material so as to be changed to aluminum hydroxide which is insoluble in benzene, whereby it does not act as a Lewis acid any more. This Patent Document 3 discloses that, as benzene, one which is completely dehydrated by drying and/or distillation treatment, is used, and in the case of employing aluminum chloride as a Lewis acid, the operation becomes cumbersome. Further, aluminum chloride has a sublimation point of 160° C. and a boiling point of 183° C., which are close to a boiling point of o-dichlorobenzene of 181° C. and a boiling point of p-dichlorobenzene of 174° C., and therefore it is difficult to separate a catalyst and a desired substance by distillation. Accordingly, industrially, aluminum chloride is not recycled but washed with water after the reaction so as to be removed and disposed of. Accordingly, in this process, a step for waste water treatment is separately required.
Patent Document 4 discloses a process for producing p-dichlorobenzene, which comprises employing, as a catalyst, an L-zeolite having potassium supported thereon, employing chlorobenzene as a raw material, and blowing chlorine gas at 70° C. It discloses that the selectivity of p-dichlorobenzene reaches 87.6% in an initial stage of the reaction. However, the conversion of chlorine in this stage is low at a level of 99.3%, and further the conversion of chlorine is reduced to a level of 80% as the reaction is repeated. As is commonly known, the reason is that, in such a zeolite method, a polychlorinated product is produced, and this product clogs pores of the zeolite, whereby the catalytic activity is deteriorated. Further, in such a production process, it is necessary to remove a zeolite catalyst by filtrating a reaction fluid after the reaction, and therefore the operation becomes cumbersome. Accordingly, a zeolite catalyst is industrially unsuitable for recycle.
As described above, as a process for producing p-dichlorobenzene by chlorinating benzene or chlorobenzene with chlorine molecules in a liquid phase, a production process employing a combination of a Lewis acid catalyst which is a halide of a metal such as iron, aluminum or antimony and sulfur or a phenothiazine analogue as a promoter, or a production process employing a zeolite catalyst, is disclosed. However, such a production process cannot satisfy both of high selectivity of p-dichlorobenzene and high conversion of chlorine at the same time, and either of them should be given up. Further, conventional art as a batch type reaction is a process with a low productivity such that a reaction fluid is withdrawn after the reaction, and subsequent to the completion of the withdrawal, a raw material and a catalyst are added to carry out chlorination reaction again. Further, such a process is not a process with which a catalyst can easily be recycled.